FIG. 1 (Prior Art) is a block diagram of primary-side regulation constant current/constant voltage (PSR CC/CV) flyback power supply 1. An alternating current (AC) 110-240 volt line voltage on input terminals 2 and 3 is rectified by a full wave bridge rectifier 4 and an associated smoothing capacitor 5 so that a rectified and smoothed rough DC voltage is present between the first and second input nodes 6 and 7. The voltage on first input node 6 is also referred to in this document as the “input line voltage” or “line input voltage” (VIN).
Power supply 1 operates by repeatedly closing and opening a switch. In the illustrated example, the switch is a bipolar transistor 8. Closing switch 8 causes a primary current to flow from node 6, through a primary 9 of a transformer 23, through switch 8, into terminal 10 of a PSR CC/CV controller integrated circuit 11, through another switch (not shown) inside the PSR CC/CV controller integrated circuit 11, and from the ground terminal 12 of the PSR CC/CV controller integrated circuit 11 to the second input node 7. When switch 8 is closed, the current that flows through primary 9 causes energy to be stored in transformer 23. When switch 8 is opened, the energy stored is transferred to the output of the power supply in the form of a pulse of current that flows through a secondary 13 of transformer 23 and through a diode 14. An output capacitor 15 is connected across output terminals 16 and 17 of the power supply. The pulse of current charges capacitor 15. In steady state operation in the constant voltage (CV) mode, switch 8 is switched to open and close rapidly and in such a manner that the output voltage VOUT on capacitor 15 remains substantially constant at a desired regulated output voltage. The output voltage VOUT is related to the voltage VAUX across an auxiliary winding 18. VAUX is divided by a voltage divider including resistors 19 and 20 so that the voltage on the voltage divider tap 21 is sensed on an FB terminal 22 of PSR CC/CV controller integrated circuit 11. PSR CC/CV controller integrated circuit 11 has an internal reference voltage generator that generates a reference voltage VREF. Controller integrated circuit 11 regulates VOUT to have the desired regulated output voltage by keeping the voltage on FB terminal 22 equal to the internal reference voltage VREF.
The current IOUT being supplied from the output of the power supply is related to the current flowing through primary 9. In the constant-current (CC) operating mode, the magnitude of the primary current is detected by detecting the voltage across the switch (not shown) inside PSR CC/CV controller integrated circuit 11. This voltage, which is the product of the current flow in the switch and the resistance of the switch, is sensed and amplified by a current sense amplifier inside PSR CC/CV controller integrated circuit 11. The output current IOUT is regulated to a desired regulated output current by keeping the peak voltage detected by the current sense amplifier equal to a reference voltage value VILIM.
In the example of FIG. 1, power supply 1 is a battery charger such as a battery charger usable to charge the rechargeable batteries of a cellular telephone. The AC line in terminals 2 and 3 are typically plugged into a wall socket to receive alternative current (AC) 110-240 VAC power. The two terminals 16 and 17 at the end of a cord are plugged into the cellular telephone. Initially, if the batteries in the cellular telephone are discharged, then the battery charger operates in the constant current (CC) mode and supplies charging current at the regulated current amount (in the present example, 1 ampere). Then, once the batteries have charged to the point that the output voltage VOUT reaches the desired regulated voltage (in the present example, 5 volts), the power supply 1 starts operating in the constant voltage (CV) mode. PSR CC/CV power supply 1 then regulates such that the output voltage VOUT stays at the desired regulated output voltage while the supplied output current decreases.
FIG. 2 (Prior Art) is a graph of output voltage VOUT versus output current IOUT. Initially when PSR CC/CV power supply 1 is operating in the CC mode, the output voltage and current operating point of the power supply lies on line 24. The point migrates vertically up line 24 as the voltage on the battery increases as the battery charges. When the point reaches corner 25, the power supply transitions to the CV mode. The point representing the output voltage and output current migrates to the left along horizontal line 26.
The graph of FIG. 2 is, however, an idealization. In an actual PSR CC/CV power supply, lines 24 and 26 are not followed. The output voltage and current points may extend away from lines 24 and 26 so much that the actual power supply operating point is outside a specified permitted operating range. In the example of FIG. 2, for the power supply to meet a particular specification and standard, the output current IOUT in the CC mode must be within plus or minus ten percent of the regulated output current of 1.0 amperes. Similarly, the output voltage VOUT in the CV mode must be within plus or minus five percent of the regulated output voltage of 5.0 volts.
There are many potential reasons that the actual power supply operating point may be outside of the specified bounds. Different units of the PSR CC/CV controller integrated circuit may be affected in different ways by the semiconductor manufacturing process used to make the integrated circuits and by the process used to package the integrated circuits. U.S. Pat. No. 6,750,640 teaches that after controller integrated circuits have been tested at “wafer sort”, that their operation from unit to unit can be affected to different degrees by the packaging process. For example, U.S. Pat. No. 6,750,640 teaches that power supply controller chips can be trimmed or adjusted after “wafer sort” and after packaging at “final test” of the integrated circuits.
FIG. 3 (Prior Art) is a replication of FIG. 1 from U.S. Pat. No. 6,750,640. At integrated circuit final test, programmable circuit connections within the integrated circuit are programmed to trim or adjust certain functions of the power supply controller chip such as over voltage threshold, under voltage threshold, external current limit, maximum duty cycle and power supply enable/disable. Although the circuit of FIG. 3 is satisfactory in certain respects, it is undesirable and/or unsatisfactory in other ways. Circuitry outside the controller integrated circuit may have electrical characteristics that are seen to differ, from power supply unit to power supply unit, such that these differences cause different units of seemingly identical mass produced power supplies to exhibit output voltage and output current operating points outside of the specified limits of FIG. 2. For example, some manufactured power supply units may operate along line 27 under certain operating conditions (see FIG. 4), whereas other units may operate along line 28 (see FIG. 4). Also, under some operating conditions, some power supply units may operate along line 29. The mass produced transformers used to make the power supplies may, for example, have primary inductances that vary from one transformer to the next. Also, mass produced power cords may have resistances that differ from each other. Such differing inductances and resistances may lead to different manufactured power supply units operating outside required bounds as illustrated in FIG. 4, even if the controller integrated circuit 11 is adjusted at final test. Moreover, it may be desired to use the same controller integrated circuit and power supply design but to attach different power cords to different units of the power supply. Some units may be made to have long cords, whereas other units may be made to have short cords. Regardless of such variations, it is desired that all the assembled power supplies supply power at the end terminals of the power cord that is within the specified five percent and ten percent bounds illustrated in FIG. 4.